Much effort has been devoted to the development of novel imaging technologies for obtaining comprehensive morphological and functional information of tissues at depth in the body. Optical microscopy normally provides high resolution but limited imaging depth in scattering media. In contrast, acoustic imaging enables deep tissue imaging with unsatisfactory resolution, limited by the low frequency used in conventional ultrasound detection. High resolution ultrasound imaging offers many new potential biomedical applications by using frequencies near or higher than 50 MHz. These include imaging internal structures of the eye, diagnosing skin pathologies, and characterizing vulnerable intravascular plaques. However, conventional piezoelectric material- based ultrasound detection is challenged by the difficulty of fabricating transducers in the high frequency range. Researchers have been seeking alternative approaches to piezoelectric ultrasound detection for decades. One approach for high frequency broadband detection uses a closed micro-cavity structure sandwiched by two optical reflectors. The low detection sensitivity of this optical technique has prevented it from widespread application in clinical settings. This program aims to address this long-standing challenge of sensitive high-frequency broadband ultrasound detection. An innovative optoacoustic sensor with an open optical micro-cavity is proposed using a photonic crystal structure in a total internal reflection configuration. This unique approach allows the sensor to be directly exposed to ultrasound signals without the attenuation of an intervening structure. Further, this configuration is compatible with highly elastic materials within the cavity while maintaining high finesse. This enables substantial increases in high- frequency ultrasound detection sensitivity and bandwidth beyond the most sensitive optoacoustic and piezoelectric sensors currently available. Successful development of this unique technology will have a great impact on high-resolution deep tissue morphological and functional imaging, applicable in a wide range of applications of biomedical research and clinical diagnosis. For example, the feasibility of using this proposed technology for intravascular high- resolution photoacoustic and ultrasound imaging will be explored. Novel Optoacoustic Sensors with an Open Cavity Configuration PUBLIC HEALTH RELEVANCE An innovative optoacoustic sensor with an open optical micro-cavity is proposed using a photonic crystal structure in a total internal reflection configuration. This unique approach will enable substantial increases in high-frequency ultrasound detection sensitivity and bandwidth beyond the most sensitive optoacoustic and piezoelectric sensors currently available. Successful development of this unique technology will have a great impact on high-resolution deep tissue morphological and functional imaging, applicable in a wide range of applications of biomedical research and clinical diagnosis.